The present invention relates to efficiency optimization and noise reduction and torque ripple reduction techniques for electric motors, and, more particularly, to an improved switched reluctance (SR) motor drive.
Switched Reluctance (SR) motors are gaining much attention due to their low cost construction and fault tolerant operation. However, two problems that have kept SR motors away from some applications is their noisy operation and torque ripple. In vehicle propulsion applications, torque ripple can result in low-speed xe2x80x9ccoggingxe2x80x9d, which is an undesirable characteristic, sometimes noticeable to drivers.
Techniques are available to reduce the noise and torque ripple of SR motors. Although these techniques are useful in many circumstances, there is room for improvement.
Two known approaches to SR motor design that reduce noise include: i) increasing the stator back iron thickness, and ii) increasing the air gap length. These design approaches to reduce motor noise tend to reduce motor torque density. As a consequence, the resulting SR motor can be bulky, heavy, and costly.
Known control techniques for reducing noise are generally based on modifying the phase de-excitation (turn-off) process during motor operation. The basic idea of these control techniques is to slow down the phase turn-off process by profiling the turn-off current tail to lower the noise. However, reduction of noise by the profiling of the tail current is obtained at the expense of motor efficiency.
One control technique presented by Pollock et al. employs noise cancellation by starting the phase de-excitation with zero voltage and applying the full negative voltage after one-half period of the stator natural resonance frequency. The acceleration of the stator back iron due to the negative voltage tends to cancel the initial acceleration. Thus, noise is reduced. However, this technique is not applicable for all motors, especially the high speed motors.
FIG. 1 illustrates a block diagram of a prior art SR motor control circuit 41 for a switched reluctance (SR) motor 11. The control circuit 41 includes a current regulator (I-REG) 46, an inverter 38, an interpolation scheme 70, a look-up table 72, current sensors 74, a position decoder 76, and an angular velocity calculator 78. Motor windings in the SR motor 11 are connected in series with inverter legs 40,42,45.
In the motor controller 41, when the speed of SR motor 11 is high, the parameters used to control SR motor 11 are phase turn-on angle, xcex8ON, and phase turn-off angle, xcex8OFF. At a low speed of operation of SR motor 11, the control parameters are phase turn-on angle, xcex8ON, phase turn-off angle, xcex8OFF and reference current, IREF. Additionally, at low speeds, because the back EMF is lower than the bus voltage, VDC, it is necessary, in addition to controlling the phase turn-on and turn-off angles, xcex8ON and xcex8OFF, respectively, to limit the phase current. Current limitation is accomplished by the current regulator (I-REG) 46 regulating the reference current, IREF, using known techniques of chopping the current.
The two primary forms of current chopping, xe2x80x9chard choppingxe2x80x9d and xe2x80x9csoft chopping,xe2x80x9d are often implemented in SR motor inverters, including those inverters similar to the prior art three-phase SR motor inverter 38, as illustrated in detail in FIG. 2. In hard chopping, both the upper and lower switches supplying a certain phase winding (illustrated in FIG. 2 as switches 48, 50 for the first phase winding 51; switches 52, 54 for the second phase winding 53; and switches 56, 58 for the third phase wind 55) are turned on and off (i.e., chopped), simultaneously. In soft chopping, one switch (e.g., 48, 52, 56) is kept on at all times, while the other switch (e.g., 50, 54, 58) is chopped. As compared with soft chopping, hard chopping provides for a greater level of control of the phase current. However, with the prior art inverter 38, hard chopping has a lower efficiency, primarily due to additional switching power losses, higher ripple current and lower power factor. Soft chopping, although it provides for higher efficiency, less ripple current, and higher power factor cannot be implemented during regenerative braking.
The reference current, IREF, at a lower speed of operation of SR motor 11, takes the shape of a square wave. The leading and trailing edges of the square wave define the phase turn-on and turn-off angles, xcex8ON and xcex8OFF, respectively, while the amplitude is the current reference, IREF. In response to this reference current, IREF, a current regulator, I-REG, turns on with full bus voltage, VDC, when the leading edge (i.e., the turn-on angle, xcex8ON) of the current reference, IREF, is encountered. The current reference, IREF, is then maintained with the chopping of the phase current, as described above. When the trailing edge of the reference current, IREF, is reached, the phase is turned off with a full negative bus voltage, xe2x88x92VDC.
At high speed, the back EMF is higher than the bus voltage, VDC. No current regulation chopping is used at high speeds, and the control is referred to as a xe2x80x9csingle-pulsexe2x80x9d mode. The control parameters at high speed are, therefore, only the phase turn-on and turn-off angles, xcex8ON and xcex8OFF, respectively. In order to build current against a high back EMF, the phase turn-on, xcex8ON, is advanced. This allows current to build before the back EMF starts to develop. The high phase inductance, of SR motor 11 holds the current for a sufficiently long time against the high back EMF, so that torque can be produced. When the turn-off angle, xcex8OFF, is reached, the phase is turned off with the full negative bus voltage, xe2x88x92VDC. In this mode, there is no chopping of phase current. Both at high speed and at low speed, there exists a unique set of control parameters that can maximize certain performance indices, such as, for example, efficiency. Noise is produced both in the low speed and in the high-speed operations of SR motor 11 during the phase turn-off stage. The high di/dt (i.e., the rate of change of current) produced by the high bus voltage, VDC, during phase turn-off sets up vibration in the stator back iron, thus generating noise.
With respect to torque ripple, current profiling is routinely done in SR motors to reduce the torque ripple, especially at low speed operation. Several techniques have been proposed to reduce torque ripple of SR motors. All of these techniques use a high bandwidth current regulator, either hysteretic or PI type, to profile the SR motor phase current such that torque ripple is reduced. A drawback of current profiling with current regulation is that it often lowers SR motor efficiency.
Accordingly it is desirable to have an improved drive for switched reluctance motors that reduces operational noise and torque ripple without sacrificing motor efficiency.
It is an advantage of the present invention to provide an improved motor drive (control) for SR motors that reduces noise, reduces torque ripple, and increases motor efficiency. Another advantage of the present invention is that it provides a motor controller that does not require phase current sensors and current regulators, such as those required by conventional SR motor drives.
According to one aspect of the present invention, a motor control includes a DCxe2x80x94DC converter coupled to an inverter. The DCxe2x80x94DC converter can be a buck converter, a boost converter, or a buck-boost converter. A capacitor can be connected in parallel across the outputs of the DCxe2x80x94DC converter supplying the inverter.
This arrangement allows the control of the DC bus voltage of the SR motor inverter. The DC bus voltage is controlled optimally to increase the efficiency of the motor. An SR motor operates more efficiently when the DC bus voltage is sufficiently lowered from the motor rated voltage such that motor phase current is in single pulse mode at all speeds and torque. Due to the reduction of the bus voltage, the current rate of change during phase de-excitation, which is the major cause of noise in SR motor, is sufficiently reduced. Hence, noise of the motor is reduced considerably.
To reduce torque ripple, the control technique profiles the bus voltage using the DCxe2x80x94DC converter to indirectly profile the phase current.
Due to the single pulse operation of the motor in the disclosed technique at all speeds, most of the switching losses are reduced for the inverter. Moreover, due to the much lower switching frequency (same as the stator electrical frequency) of the inverter, it is possible to replace fast insulated gate bipolar transistors (IGBTs) (fast IGBTs are needed to improve the current control bandwidth) with slower IGBTs or other switching devices, which usually have much lower saturation voltage. Thus, inverter conduction losses can also be reduced. Furthermore, lower voltage single pulse operation of the motor exhibits higher power factor than the conventional chopping mode of control. Thus, machine and inverter losses are further reduced.